Only test permission failures if we're not root
Only test permission failures if we're not root

or, more generally, if we're on a system where permissions are being enforced

Closes: #1979

lmcro/letsencrypt,twstrike/le_for_patching,bsmr-misc-forks/letsencrypt,mitnk/letsencrypt,kuba/letsencrypt,dietsche/letsencrypt,jsha/letsencrypt,brentdax/letsencrypt,bsmr-misc-forks/letsencrypt,TheBoegl/letsencrypt,thanatos/lets-encrypt-preview,lmcro/letsencrypt,stweil/letsencrypt,wteiken/letsencrypt,jtl999/certbot,jsha/letsencrypt,DavidGarciaCat/letsencrypt,thanatos/lets-encrypt-preview,VladimirTyrin/letsencrypt,TheBoegl/letsencrypt,VladimirTyrin/letsencrypt,dietsche/letsencrypt,wteiken/letsencrypt,brentdax/letsencrypt,letsencrypt/letsencrypt,stweil/letsencrypt,DavidGarciaCat/letsencrypt,kuba/letsencrypt,jtl999/certbot,twstrike/le_for_patching,mitnk/letsencrypt,letsencrypt/letsencrypt